The present invention relates to a fluid pressure control system for controlling the various speed stages of an automatic transmission for a vehicle such as an automobile and, more particularly, to a fluid pressure control system for regulating the back pressure of an accumulator.
As is well known in the relevant art, the vehicular automatic transmission uses a gear mechanism such as a planetary gear set composed of three elements, two of which are used for an input and an output with the remaining one being fixed. With this construction, speed stages or ranges such as gear ratios including reverse and plural forward stages are set by switching the fixed and meshing states of those gear elements through friction engaging means such as a clutch and a brake. The engagement and release of the friction engaging means are controlled by fluid pressures. The vehicular automatic transmission of this type is disclosed in various references such as Japanese Patent KOKAI No. 61-149657 directed to a gear train or "Manual of New Car -- Toyota Corona FF" (issued by Toyota Jidosha Kogyo Kabushiki Kaisha) directed to a gear train including a fluid pressure control circuit.
In the automatic transmission, as is disclosed in these references and known in the art, transmissions of torques in the individual speed stages are effected through friction engaging means so that the load to be exerted upon the engaging means highly differs in dependence upon a throttle opening and a vehicular running speed. This makes it necessary to reduce the gear shift shock by regulating either the pressure of a working fluid to be exerted upon the friction engaging means in accordance with the throttle opening or the back pressure of an accumulator which is disposed midway of a fluid passage leading to the friction engaging means, thus maintaining the durability of this means.
In the invention disclosed in the above-specified Japanese Patent KOKAI No. 61-149657, for example, a computer-controlled back pressure regulator (e.g., a duty valve) is provided for processing various pieces of information such as the throttle opening, the running speed or the kinds of speed stages to send signals to the back pressure regulator so that the back pressure of the accumulator may be suited for all the speed stages and the vehicles.
In the invention disclose in Japanese Patent KOKAI No. 58-152961, on the other hand, noting that the fluid pressures for completing the engagement of the friction engaging means for high- and low-idling operations are different, the accumulator back pressure is boosted for the high-idling operation to reduce the gear shift shock which the speed stage is manually shifted from the neutral range to the drive or reverse range.
Moreover, the invention disclosed in the specification of U.S. Pat. No. 4,428,258 notes that the engine output torque is remarkably low for a small throttle opening and gently rises for a large throttle opening so that it is not linearly proportional to the throttle opening. Control is made to drop the fluid pressure for engaging the friction engaging means for the large and small throttle openings.
In Japanese Patent KOKAI No. 61-130653, still moreover, there is disclosed a system which is operative to detect the magnitudes of the causes of the gear shift shock in terms of the throttle opening and has a control valve for controlling the accumulator back pressure in accordance with those magnitudes.
Here, the switching of the gear ratios by the gear transmission mechanism of the automatic transmission is performed by engaging or disengaging a plurality of friction engaging means. Of these, some means is engaged at a plurality of gear ratios. For example, the direct clutch (or C.sub.2 clutch), as is disclosed in the above-specified Japanese Patent KOKAI No. 61-149657 or "Manual of New Car --Corona FF", is engaged in the two cases in which the manual valve is set in the reverse (R) stage and in which a third or higher speed stage is set in accordance with the throttle opening and the vehicular running speed. The accumulator belonging to such friction engaging means of dual stage type is generally tuned in the prior art to the characteristics required for shifting up the speed stage from 2nd to 3rd, taking preference of avoiding the gear shift shock while the vehicle is running. In other words, the back pressure of the accumulator, the area of the back pressure or the strength of a spring is adjusted to become suitable for the upshift to the 3rd speed. For the shift to the reverse range, however, the aforementioned tuning will make the timing premature for the engagement of the direct clutch, thus causing the disadvantage of the gear shift shock.
On the contrary, the aforementioned control for dropping the working fluid pressure with a small throttle opening (as is disclosed in the specification of U.S. Pat. No. 4,428,258) is accomplished with reference to the throttle opening but not the difference between the selection of the reverse range and the upshift to the 3rd speed. This makes it difficult to eliminate the aforementioned gear shift shock at the instant when the reverse range is set.
In the control of raising the accumulator back pressure in a high-idling operation (as is disclosed in Japanese Patent KOKAI No. 58-152961), on the other hand, the accumulator back pressure in a low-idling operation is made different from that in the high-idling operation. In the case of the reverse range, however, it is the current practice to boost the accumulator back pressure so as to increase the engaging force of the friction engaging means such as the clutch because the torque to be transmitted by that means is high. If the reverse range is to be established in the low-idling operation, the accumulator back pressure is boosted to a higher level than that of the case in which the gear is shifted to another speed stage. As a result, the method disclosed in the Japanese patent KOKAI No. 58-152961 is accompanied by the gear shift shock because the engaging timing of the friction engaging means for the setting of the reverse range is premature.
On the other hand, the system (as is disclosed in the Japanese Patent KOKAI No. 61-149657) controls the accumulator back pressure for all the conditions by means of its computer. In the reverse range, the system boosts up the accumulator back pressure as the torque to be transmitted is high, so that it cannot always reduce the shock upon shifting to reverse without fail. In addition, this system is troubled by other problems of complicated construction or high production cost, which are caused by adding electric devices such as a computer or an electromagnetic valve.
On the other hand, the control system disclosed in the Japanese Patent KOKAI No. 61-130653 is one controlling the accumulator back pressure in accordance with the magnitudes of the causes for the gear shift shock such as the throttle opening. In case the reverse range is to be set, the fluid pressure for engaging the clutch for the reverse and forward ranges is throttled by means of an orifice to slowly raise the fluid pressure. This equalizes the rises in the fluid pressure for setting the 3rd forward and reverse ranges. As a result, the rising characteristics of the fluid pressure have to be tuned suitably for setting either the 3rd speed or the reverse range. As a result, the tuning becomes unsuitable for either of the settings, followed by the gear shift shock.
As has been described above, the prior art is troubled by the serious problem that the gear shift shock is higher upon shifting to reverse than upon shifting to each of the forward speed stages, because the accumulator back pressure for reverse is either as high as or positively higher than that for the 3rd forward stage, for example. If, on the other hand, the accumulator back pressure is dropped to reduce the gear shift shock for reverse, there arises another problem that the shock for the 3rd forward speed stage is increased.